Improve for recognising shell

- Add screenshot for every shell listed (when dealing with text editors take screenshot when `git commit` call this editor).
- Move "how to exit" for close of recognising
- Add R shell

@mikej888 could you review this one?
This page isn't really a session, more of a reference. So change to:
"On this page you will find tips to discover where you are and how to exit if you are not where you want to be."